Papermaking is a sophisticated operation involving massive and very expensive machines. These machines are increasingly running at higher speeds, meaning that their overall efficiency must be very high, and in particular, the efficiency of their sub-components must also be very high. The papermaking process requires that water be removed from the initial pulp fiber solution as the paper is formed. The pulp fiber solution, once in the drying section of a papermaking machine, is referred to as the paper web. The paper web is supported as it travels across the machine following a path during which moisture is progressively removed therefrom. The support is provided by endless sheets of porous fabric, felts, wires or other water and gas permeable support means, all of which are generically referred to as the “felt or felts” in the description and appended claims.
The paper web travels from what is referred to as the wet end of the machine to the dry end thereof. In its path, the paper web runs over numerous heated drying cylinders where moisture is evacuated therefrom either by direct evaporation or transfer of moisture to the felts or to the surface of the drying cylinders. A network of ventilator devices is used throughout the drying section in order to inject heated dry air at numerous locations and promote the removal of moisture from the papermaking machine. At the end, the machine outputs the resulting paper, which is then generally reeled to be shipped elsewhere.
Papermaking machines can be built according to numerous possible configurations. One configuration in particular is the twin-wire draw, where the papermaking machine comprises two superposed rows of axially-parallel and horizontally disposed heated drying cylinders. The paper web runs in a serpentine or zigzag path where it defines loops by alternating between the two rows of drying cylinders as it advances along the drying section. The paper web is being supported in most of its path with the assistance of the felts. There is generally one felt for each row of drying cylinders. Each felt presses the paper web on a portion of the surface of the drying cylinders of the corresponding row. Each felt also runs over a felt roll between each pair of adjacent drying cylinders of a same corresponding row. The felt rolls are located deep in the space between the two adjacent drying cylinders. This configuration allows maintaining the paper web in supporting contact with the felt as long as possible. The felt rolls essentially redirect a felt to the next drying cylinder of the same row.
It should be noted at this point that the terms “roll or rolls” and “cylinder or cylinders” are synonyms since both are elongated member with a circular cross-section, the only distinctions in the present context being that the drying cylinders are generally much larger in diameter than the felt rolls and are heated by appropriate means that are well known in the art. The segregated use of the terms in the text is only for the purpose of clarity. The “cylinder or cylinders” are sometimes referred to as “drum or drums” in other documents.
Traditional drying cylinders and felt roll arrangements feature the felt roll being positioned intermediate the axis of rotation of adjacent drying cylinders, thus halfway between two adjacent drying cylinders. Inherent in these arrangements is the fact that the paper web is repetitively unsupported wherever the felt separates from the paper web to pass over a felt roll.
In order to increase the speed of papermaking machines, it was desirable to reduce the length of unsupported paper web because of the risks of rupture thereof, particularly near the wet end where the paper web is weaker. One solution to this problem was to move the axis of rotation of the felt rolls backwards, more particularly towards the wet end of the machine, as illustrated in FIG. 1. This offset configuration has resulted in the felts and the paper web being in contact longer, thus minimizing the length of unsupported paper web as it travels from one drying cylinder to another.
The offset configuration of the felt rolls has also resulted in creating what is known as offset pockets. In FIG. 1, the pocket spaces (12) are identified as hatched areas. Each pocket space (12) is situated between three successive drying cylinders (20) over which consecutively run a paper web (14). There are thus many offset pockets, such as top pocket (12a) and bottom pocket (12b) since there are many groups of three successive drying cylinders (20). A pocket space (12) may be roughly defined as the space limited by a felt (16) between two successive drying cylinders (20) of a same row, a first draw of paper web (14) from a first drying cylinder (20) to the next drying cylinder (20) of the other row, a second draw of paper web (14) between that second drying cylinder (20) and a third successive drying cylinder (20) on the same row as the first one, and the free surface of the second drying cylinder (20). The pocket spaces (12) are only open at each side of the machine. It should be noted that the pocket ventilators have been omitted from FIG. 1 for to simplify the drawing.
An offset pocket space (12) is situated between each group of three axially-parallel drying cylinders (20) over which consecutively runs the paper web (14). Among these cylinders (20), the first and third ones (20a, 20c) are vertically spaced from the second one (20b), as shown in FIG. 1. This is due to the fact that there are usually two superposed rows of cylinders (20) in a papermaking machine, more particularly a lower row and an upper row. Since the paper web (14) follows a serpentine or zigzag path across the drying section, there is a plurality of successive pocket spaces (12) in a papermaking machine. The first and third cylinders (20a, 20c) of a given group will be either on the lower or upper row, while the second cylinder (20b) belongs to the opposite row.
The paper web (14) is pressed against the corresponding first (20a) and third (20c) cylinders by a felt (16) which further runs over a felt roll (26) having a rotation axis which is parallel to that of the cylinders. The felt roll (26) is disposed between the set of three cylinders in an offset position, more particularly in a position which is closer to the first cylinder (20a) than the third cylinder (20c), as shown in FIG. 1.
Unfortunately, ventilating offset pocket spaces is more difficult than in symmetrical pockets spaces configurations. This results from the reduction of the length of felt which does not support the paper web on the side upstream of the felt roll. Since the felt is permeable to air and the paper web is not, the conventional dry heated air ventilators which were hitherto provided to ventilate the felt cannot be used the same way since air cannot be blown through the felt when it is supporting the paper web. All of this has resulted in decreased air flow into offset pocket spaces, thus a decrease in the efficiency of the ventilation.
An example of a pocket ventilator previously known in the art is disclosed in U.S. Pat. No. 5,074,278. It illustrates a traditional symmetrical arrangement of pocket spaces. This patent is hereby incorporated by reference.
The geometry of a pocket and the natural air currents generated in the pocket space by the moving paper web and felt, as well as the rotation of the drying cylinders and the felt rolls, are key factors which increase the difficulty in ventilating a pocket space. Air introduced in a pocket space has a natural tendency to follow the movement of the felt and also tends to be trapped in a cleft defined by the felt and a downstream drying cylinder. This cleft, known as the closing nip, is a zone of positive air pressure where air tends to flow through the felt to evacuate the pocket space. Furthermore, a zone of negative air pressure is created in a cleft defined where the paper web leaves the felt roll on its way to the next drying cylinder. Some air evacuates the pocket space to satisfy the negative pressure created therein at the opening nip. However, these natural air currents do not provide an adequate ventilation of the pocket space as they do not effectively sweep the pocket space. As a result, air becomes entrapped therein, thereby increasing the humidity level within the pocket space and decreasing the overall drying capacity of the papermaking machine.
U.S. Pat. No. 6,725,569 provides a device for ventilating an offset pocket space as illustrated in FIG. 2. However, trying to introduce air at a first cleft (34), defined where the felt and paper come in contact with the felt roll, is not always desirable as it involves a detachment of the sheet of paper (14) from the felt (16). It appears that introducing air at the first cleft (34) has a tendency to introduce sheet fluttering or sheet instability. To overcome such problem, the operator typically closes the associated internal damper 46A or the air passage at 46A to reduce or cancel the amount of air being introduced by air jet 51 at the first cleft 34.
There is still a need in the art for a pocket ventilator device and method.